1. Field of the Invention
The present invention relates to a method and related apparatus of handling a variable of a Radio Link Control (RLC) reset procedure during receiver-side-only re-establishment in a wireless communications system, and more particularly to a method and related apparatus for keeping chances for retransmission of RESET PDU (protocol data unit) after receiver-side-only re-establishment.
2. Description of the Prior Art
The third generation (3G) mobile communications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA can provide high frequency spectrum utilization, universal coverage, and high quality, high speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse flexible two-way transmission services and better communication quality to reduce transmission interruption rates.
Taking a signal communications protocol standard set forth by the 3rd Generation Partnership Project (3GPP) as an example, targeting the Access Stratum (AS), the 3G mobile communications system defines various protocol stacks, such as Radio Resource Control (RRC), Radio Link Control (RLC), Media Access Control (MAC), Packet Data Convergence Protocol (PDCP), and Broadcast/Multicast Control (BMC). In addition, the 3G mobile communications system also provides different levels of transmission quality, and can operate in different corresponding modes according to different transmission quality requirements, such as: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). TM is appropriate for use in services with high requirements for real-time transmission, UM is appropriate for use in services with requirements for real-time transmission and packet sequencing, and AM is appropriate for use in services with low requirements for real-time transmission, but high requirements for data accuracy.
In AM, the RLC layer combines a transmitter side and a receiver side. The transmitter side and the receiver side each process transmission and reception through the RLC layer, and both sides can share system resources. In some circumstances, the RLC layer must be re-established, e.g. when changing a PDU size. The prior art re-establishes the RLC layer by re-establishing the transmitter side or the receiver side.
Directed at operations corresponding to re-establishing the RLC layer, a communications protocol specification established by the 3GPP (3GPP TS 25.322 V6.7.0 (2006-03), “Radio Link Control (RLC) protocol specification (Release 6)”) is already described in detail. According to section 9.7.7 of the aforementioned RLC protocol specification, the prior art has disclosed operations of specified control PDUs and Timers when only the receiver side is re-established. However, the prior art does not teach how to handle a reset state variable, VT(RST).
According to the aforementioned RLC protocol specification, the reset state variable VT(RST) is used to count the number of times a RESET PDU is scheduled to be transmitted before a reset procedure is completed. The sender shall increment VT(RST) by 1 when a STATUS PDU or a piggybacked STATUS PDU reported from the receiver includes “erroneous Sequence Number”. If the reset state variable VT(RST) equals its maximum value, the value of the protocol parameter MaxRST, the sender shall terminate the ongoing RLC reset procedure, stop the timer Timer_RST if it was started, and indicate unrecoverable error to the upper layer. Otherwise, if the reset state variable VT(RST) is smaller than the protocol parameter MaxRST, the sender shall output a RESET PDU to start RLC reset procedure. In addition, the initial value of the reset state variable VT(RST) is 0. The reset state variable VT(RST) is only reset upon the reception of a RESET ACK PDU, i.e. VT(RST) is not reset by the RLC reset procedure.
In the prior art, when only the receiver side of an RLC entity is re-established, the current RLC reset procedure is aborted, and then a new RLC reset procedure is started. Under such circumstance, the status variable VT(RST) of the aborted RLC reset procedure is not reset. If the status variable VT(RST) is near the maximum allowed value, say MaxRST-1, then there is only one chance for the sender to transmit the RESET PDU and to receive the RESET ACK PDU. If either one of them gets lost during radio transmission, the reset state variable VT(RST) is not reset but incremented by 1, meaning that VT(RST)=MaxRST. As a result, the sender terminates the ongoing RLC reset procedure, and indicates unrecoverable error to the upper layer. The RLC entity will be re-established again unnecessarily.